Hydrogen sulphide (H2S) is a flammable, irritating, corrosive, bad-smelling gas with very high toxicity. Toxicity of the hydrogen sulphide is comparable with hydrogen cyanide, which is considered as a broad-spectrum poison. Hydrogen sulphide can affect different parts and systems such as skin, eyes and throat in the human body, depress the nervous system and eventually cause death. Hydrogen sulphide occurs naturally in the environment, but ultra low levels can be tolerated because the human body can possess a number of enzymes that are able to perform the conversion through oxidation of hydrogen sulphide to sulphate. Thus, the detection of H2S is a major challenge for air quality monitoring.
It is important to continually sense the hydrogen sulphide to provide safeguards for employees who work in areas like petrochemical and fuel refinery industry where exhibits H2S is exhibited. The detection of hydrogen sulphide is also very beneficial for the biomedical field, especially for determination of H2S content in mouth air and for diagnosis in dentistry. Semiconductor oxides play a significant role for H2S sensing. Tin dioxide-based materials such as pure SnO2, CuO—SnO2 and SnO2—Ag2O can easily sense H2S in air. Copper oxide can be a best promoter for the SnO2-based hydrogen sulphide sensors; however, such sensors exhibit maximum sensitivity at elevated temperatures, (i.e. around 150° C.). At this elevated temperature, irreversible reactions can take place between the gas and the sensing layer, which affects the long-term stability of the sensor.
The majority of prior art gas sensors utilize a thin solid film on a SAW/BAW device to overcome the aforementioned drawbacks. In such gas sensors, gas molecules are absorbed onto the surface of solid film due to interactions such as hydrogen bond, electrostatic, pi-pi stacking, Van Der Waals interactions and host-guest relationship. Therefore, the propagation velocity of the SAW/BAW acoustic waves can be alerted as a function of the gas and eventually a shift in the phase or resonance frequency of SAW/BAW devices induced. Sputtered inorganic film based on activated tungsten trioxide materials, (e.g. pure tungsten trioxide, doped tungsten trioxide with iridium, gold and palladium), can be developed to form a sensitive film for hydrogen sulphide detection. Such thin films exhibit a good sensitivity toward hydrogen sulphide, but unfortunately the temperature still remains too high, (i.e. around 130° C.).
Recently, calixarenes have been used to achieve organic compounds with versatile applications such as gas sensors, solar cells, batteries, antistatic coatings, electro-luminescent devices, electrodes, nonlinear optical devices, transistors, etc. Therefore, sensitive layers can be made using calixarenes and related compounds for H2S detection. But, the immobilization of calixarenes and related compounds at the surface of a piezoelectric substrate can require laborious synthesis of calixarene derivatives, which contain reactive functional groups for polymerization to the surface of the piezoelectric substrate. Such functional groups are typically linked to the lower rim of the calixarene or other related compounds via flexible tethers.
A need therefore exists for an improved method for design of a sensitive layer with high sensitivity, which enables hydrogen sulphide detection at room temperature with SAW/BAW devices. Such an improved method is described in greater detail herein.